Study On Polarization Property And Unpredictability Of Vcsel-Based Chaotic System

With the requirement of constructing broadband, ubiquitous, integrated and safety network infrastructure for the new generation of information technology, as well as the increasing requirement of communication bandwidth, optical fiber communication has been a dominant transmission technology owing to its capability of providing wide transmission bandwidth. However, as the calculation performance has also been enhanced significantly, the eavesdropper can attack the communication system readily, and thus seriously threatens and reduces the communication security. Optical chaotic communication, as a novel secure communication technology, has received more and more attention, as it is compatible with the existing communication network, and represents an additional encoding scheme from the physical layer of the network, which aims at reinforcing the standard cryptographic architecture. As commonly employed optical sources, laser diodes (LDs) are key components in optical communication systems, and determine to some extent the development of optical fiber communication technology. Compared with the conventional edge-emitting laser (EEL), vertical-cavity surface-emitting laser (VCSEL) exhibits several advantages, such as low threshold current, low cost and easy production of two-dimensional arrays. In particular, its rich polarization property can be utilized to enhance the security of optical chaotic communication system. Due to the great demand and hotspot of the secure optical communication and optical signal processing, this work concentrates on the VCSEL-based chaotic system. The aims are to explore novel tool to realize and control the polarization switching (PS) in VCSEL. and hence, to expand the application of VCSEL-based all-optical signal processing, such as optical switching and optical buffer memory. Besides, we propose quantifier to evaluate quantitatively the unpredictability degree of chaotic signals in VCSEL and try to design novel and useful scheme to generate unpredictability-enhanced chaotic signals and to realize security-enhanced chaotic communication systems, which can promote the VCSEL-based chaotic communication systems, and hence, are important from both theoretical and practical perspectives.This thesis is supported by the National Natural Science Foundation of China (Chaos synchronization and polarization properties in mutually-coupled LDs with multi-channel time-varying injection; High-speed random number generators based on chaotic LDs and application in communication system), by the Specialized Research Fund for the Doctoral Program of Higher Education of China (Theoritical study on control and application of nonlinear dynamics in VCSEL-based system), by the Fundamental Research Funds for the Central Universities (Study on detection and evaluation of chaos synchronization in LD-based chaotic system), and by the Funds for the Excellent Ph.D. Dissertation of Southwest Jiaotong University (Study on polarization and unpredictability properties in LD-based chaotic communication system). The thesis concentrates on the polarization properties of VCSEL, as well as the effects of polarization properties on the unpredictability degree and communication performance. The main contents include the following parts. At first, we investigate the polarization properties in detail, and explore a novel tool to control the PS. Then, we study quantitatively the unpredictability degree of chaotic signals generated by VCSEL subject to different feedback schemes, and analyze the effect of polarization mode competition on the unpredictability of chaotic signals. Following, we explore the generation of unpredictability-enhanced chaos in VCSEL, and discuss the concealment of time delay signature. Besides, we also discuss the influence of unpredictability on the synchronization quality and communication performance. At last, based on all-fiber setup, we propose and experimentally realize general schemes to enhance the unpredictability of chaotic signals in long wavelength LDs, which can be utilized in both EEL-based and VCSEL-based chaotic systems.The investigations on the PS properties and unpredictability degree of chaotic VCSEL, as well as the synchronization quality and communication performance of VCSEL-based chaotic systems are performed based on rate equation model. The numerical studies are carried out by Rung-Kutta algorithm. In particular, the quantitative analyses on the unpredictability degree are performed by permutation entropy (PE). The unpredictability properties of long wavelength LDs are further investigated both experimentally and numerically. The innovative results of this dissertation are:16pieces of SCI-indexed papers have been published, one utility model has been authorized, and two national invention patents have been applied. The innovation of this thesis can be presented as follows.Firstly, we propose a novel feedback scheme, the variable-polarization optical feedback (VPOF). By introducing VPOF in the external cavity of VCSEL. we provide a new tool to realize PS. By statically varying the angle of RP, the PS can be realized even for fixed injection current and feedback strength. Besides, the PS point can be controlled by different injection current or feedback strength. To quantify the degree of polarization in VCSEL, we further introduce the fractional polarization based on Stokes parameters. The representations on the Poincare sphere are also presented to observe the polarization evolution during the PS process. If the polarizer angle of RP is varied continuously, the bistable PS can also be obtained in VCSEL with VPOF. Moreover, the hysteresis region can be controlled by the sweep rate of the polarizer angle of RP, the feedback strength and the feedback delay. Based on VCSEL with VPOF, we further construct mutually-coupled VCSELs system, and compare the polarization properties for two different injection schemes, the parallel-polarization optical injection (PPOI) and orthogonal-polarization optical injection (OPOI). It is found that richer polarization properties can be achieved under the condition of OPOI. the frequency detuning induced PS can also be obtained. This work provides a new tool to obtain controllable PS in VCSEL, which expands the application of VCSEL-based all-optical signal processing.4papers have been published, i.e.,[Appl. Opt.,48(27):5176,2009],[J. Opt. Soc. Am. B.,27(3):476,2010],[J. Opt. Soc. Am. B.,27(12):2512,2010],[IEEE J. Sel. Top. Quantum Electron.,(accepted2012)].Secondly, we propose to adopt unpredictability degree as a key quantifier for chaotic signals, and with the aid of PE we evaluate quantitatively the unpredictability degree of chaotic signals in VCSEL with conventional polarized optical feedback, which compensates for the limitations of the previously qualitatively analysis techniques from the time domain and frequency domain. Based on PE, we investigate in detail the unpredictability of chaotic signals in VCSEL subject to parallel-polarization optical feedback (PPOF), as well as in VCSEL subject to polarization-rotated optical feedback (PROF). The effects of feedback strength, feedback delay, as well as injection current are discussed and compared in detail. In addition, for the first time, we discuss the influence of polarization mode competition on the unpredictability degree of chaotic signals generated by VCSEL subject to PROF. It is shown that, when two polarization modes have comparable intensities, the PE values for XP mode. YP mode, and total output are close to each other, but the PE value for total output is the highest one when dominant polarization mode exists. This work provides effective tool to quantify unpredictability degree of chaotic signals, which gives valuable information for choosing proper chaotic carrier in optical chaotic communication systems.2papers have been published, i.e.,[IEEE J. Sel. Top. Quantum Electron.,17(5):1212,2011],[Opt. Lett.,36(3):310,2011].Thirdly, considering the rich polarization properties, we unveil the conditions to realize unpredictability-enhanced chaos and time delay concealment in VCSEL with VPOF. For given feedback strength and injection current, the unpredictability-enhanced chaotic signals can be obtained in VCSEL by properly selecting the polarizer angle of RP. By adopting VCSEL with VPOF as master laser, we further propose dual-path-injection (DPI) to a slave VCSEL, to enhance significantly the unpredictability degree of chaotic signals generated in salve VCSEL. Besides, we also investigate quantitatively the concealment of time delay signature for chaotic signals generated by VCSEL with VPOF. In particular, we propose two quantifiers, the peak to mean ratio based on the auto correlation function, and the valley to mean ratio based on the PE function. Based on the two proposed quantifiers, we find that the time delay signature can be well concealed by properly choosing the polarizer angle of RP, which prevents an eavesdropper to retrieve the correct time delay of the chaotic systems, and thus enhances the security of chaotic communication systems to some degree. This work proposes a novel scheme to enhance the unpredictability degree and conceal the time delay signature of chaotic signal generated in VCSEL.3papers have been published, i.e.,[IEEE J. Lightw. Technol..29(14):2173,2011],[Opt. Comm.,284,5758,2011],[Chin. Phys. Lett.,28(1):014203,2011].Fourthly, the synchronization quality of unpredictability-enhanced chaos in VCSEL for different injection schemes are discussed and compared in detail, and the communication performance are also discussed. For PPOI, when the injection strength is sufficiently large, the high quality synchronization between the transmitter and receiver VCSELs can be achieved for both low-unpredictability and high-unpredictability chaos.While for VPOI, only the low-unpredictability chaos can be well synchronized, but the synchronization between unpredictability-enhanced chaos can not be realized even for strong injection. Based on PPOI, and by adopting chaos shift keying and chaos modulation techniques, the security-enhanced chaos communication is realized. This work numerically realizes the successful message encoding and decoding based on synchronization of unpredictability-enhanced chaos, which expands the application of VCSEL-based security-enhanced chaotic communication system.5papers have been published, i.e.,[Opt. Lett.,36(17):3497,2011],[IEEE J. Quantum Electron.,47(10):1354,2011],[IEEE Photon. Technol. Lett.24(15):1267,2012],[Opt. Comm.,285(24):5293,2012],[Opt. Laser. Technol.,42(4):674,2010].Fifthly, we experimentally design and realize two general schemes to enhance the unpredictability degree of chaotic signals generated by long wavelength LDs based on the all-fiber setup. Based on the conventional master-slave injection configuration, we propose DPI from single master laser into a slave LD (S-LD). The unpredictability and bandwidth enhancement are observed experimentally. Numerical results further show that, the unpredictability degree and bandwidth of chaotic signals for S-LD with DPI are much higher than those for S-LD with single-path-injection, and the parameter regions contributing to wideband unpredictability-enhanced chaotic signals are greatly broadened. Moreover, positive frequency detuning is preferred to further enhance the unpredictability and chaotic bandwidth. On the other hand, we also introduce dual-chaotic-optical injection (DCOI) from two different master lasers into S-LD. It is found that, by choosing different positive frequency detuning, higher unpredictability degree can be achieved. This work achieved wideband unpredictability-enhanced chaotic signals in LDs, which are highly desirable for security-enhanced chaotic communication systems, as well as high speed random number generators based on chaotic LDs.2papers have been published, i.e.,[IEEE J. Quantum Electron.,48(8):1069-1076,2012],[IEEE Photon. Technol. Lett.,(accepted2012)].In summary, consdiering the security of optical communication, this thesis concentrates on the polarization and unpredictability properties in VCSEL-based chaotic system. We proposed a novel feedback scheme, i.e., VPOF. to realize PS in VCSEL, which promotes the application of VCSEL-based optical signal processing field. Besides, we firstly adopt PE to evaluate quantitatively the unpredictability degree of chaotic signals, and to analyze the effect of polarization mode competition in VCSEL on the unpredictability degree, which provides an effective quantifier for selecting carrier in chaotic system. Furthermore, based on VCSEL with VPOF. unpredictability-enhanced chaos are generated and well synchronized, and successful message encoding and decoding are also achieved. We experimentally and numerically demonstrate two general schemes to enhance the unpredictability degree of chaotic signals in LDs, which are extremely useful for the applications of LDs-based security-enhanced chaotic communication systems and random number generators.